The Walkie-Talkie: battle of the bulge on Fenchurch Street

It has upset Unesco and been called 'brutally dominant' by English Heritage. But will a dramatic free-to-all rooftop garden redeem this new London skyscraper? As the building tops out, Rafael Viñoly defends his creation

'A party city in the sky' ... 20 Fenchurch Street, AKA the Walkie-Talkie, looms over the City of London. Photograph: Chris Ratcliffe/Bloomberg/Getty

"Big thinkers," declares the billboard at the foot of 20 Fenchurch Street, "need big floors." Looming over this declaration, swelling upwards and outwards over its neighbours, stands Rafael Viñoly's latest skycraper, a bulging new addition to the City of London. Nicknamed the Walkie-Talkie, the self-styled "building with more up top" is proof that form follows finance: 20 Fenchurch is practically a diagram of the forces that created it.

Soaring over the skyline like a broad-shouldered banker, the building flares outwards so that its biggest floors can be sited at the priciest upper levels: a monument to the logic of the office rental market and the peculiar whims of the City planning authority. Although it won't be fully completed until April 2014, 20 Fenchurch topped out this week; with its structural frame now complete, and half-dressed in a pinstriped suit of glazing and louvers, its hulking presence is already being felt.

"Every building is an occupation of the skyline," says Viñoly, speaking on the phone from New York, where he is based. "But most don't give anything back." The architect, who has spent his career trying to marry public amenity and commercial reality, is referring, in part, to the fact that the summit of his 160m colossus will boast a three-level "skygarden" that's open to everyone – for free. Comparing this to the £25-a-ticket viewing gallery of the Shard, just across the river, Viñoly says: "You have to ask what the public gets by accepting a further intrusion on the city. The possibility of offering an urban experience at a height is pretty remarkable."

The Walkie-Talkie, which in fact looks more like a brimming pint glass, is the most incongruous thing to have appeared on the London horizon for some time – quite an achievement in a city that has recently seen the erection of the Strata, a three-eyed Mordor tower at Elephant and Castle with its trinity of rooftop turbines; and a giant shard of glass at London Bridge. Meanwhile, coming soon are the Leadenhall Building (AKA the Cheesegrater) in the City, and a rolled-up napkin at Bishopsgate (if the stalled Pinnacle build ever restarts).

But while these buildings are the product of a vague planning concept, which promotes building tall over transport hubs and at the central City cluster, the Walkie-Talkie is a bizarre anomaly that has shouldered its way into being. First proposed in 2004, the design was criticised by both English Heritage and Unesco. The former declared it an "oppressive and overwhelming form" and a "brutally dominant expression of commercial floor space"; the latter threatened to add the Tower of London to the World Heritage in Danger list, because of the detrimental impact the skyscraper would have on its setting. It was bitterly contested by its neighbours over their right to light, and subjected to a public inquiry over heritage concerns. Yet, ever in thrall to the intoxicating cocktail of big business, star architects and a quirky nickname, the planners cheerfully beckoned the scheme through.

"Our first reaction was that it's not an appropriate place for a tall building," says Peter Rees, chief planner of the City, who has presided over its bold, vertical evolution since 1985. His mind was changed by the idea of a skygarden – and the views it would provide over the huddle of towers that have sprung up during his reign. "We came to think of it as the figurehead at the prow of our ship," he says. "A viewing platform where you could look back to the vibrancy of the City's engine room behind you."

'Bowing to the river' ... what the completed building will look like

Having been allowed up to the 28th floor, I can confirm that the view is indeed thrilling: short of hiring a helicopter, it's the only place where you can look directly across at the Shard and back to the City, where the gleaming ducts of Richard Rogers' Lloyds building are now framed by the cross-bracing of his Cheesegrater; and where the shaft of Norman Foster's Gherkin is echoed in the concave shoulders of his Willis building. It is a compelling tableau, charting the evolution of British hi-tech. And it makes you realise what a choked thicket of competing aspirations the City has become.

Besides the view, the big attraction for developer Land Securities' prospective tenants (mostly insurance firms) is the expansive, uninterrupted floorspace, which allow companies to pack people in at a higher density: one person for every eight square metres, compared to the City average of 11 or 12. You do wonder quite how comfortable such battery-farm levels will be, with some workers sitting up to 20m away from windows. But the primary lure – for wooing tenant and planner alike – is clearly the skygarden, particularly for an industry that is all about face-to-face dealings.

"The secret of the City's success is having places to gossip," says Rees. He reels off a list of the latest rooftop watering holes he has eased into being, from the terraces of One New Change, which enjoy views across to the dome of St Paul's, to the panoramic restaurants at the peak of the Heron Tower, as well as the forthcoming swimming pool and bars on the summit of the old Midland Bank, the latest branch of private members' club Soho House. "We are taking every opportunity to create the party city in the sky," he says. "It's very important to our business offer that people can party as close to their desks as possible." Rees is open about his vision for the City as a playground for twentysomethings, arguing that London's nightlife is a great way to attract the best minds.

The Walkie-Talkie's skygarden will doubtless become a place where huge deals are brokered over glamorous cocktails; but the fact that this floating Babylon will require advance booking means it's questionable how much of a public space it will actually be. An earlier attempt to create "London's living room" at the top of Foster's City Hall has been closed off and used for private events ever since it was completed.

Viñoly describes his new 63,000 sq m tower in modest tones, referring to the way the building looms southward towards the Thames as a form of deference: he calls it "bowing to the river". As for building something so tall in this location, he maintains that 20 Fenchurch is actually "stepping down from cluster", arguing that it's a peripheral foothill to the City's taller central peaks. And, in response to questions of heritage and proximity to the Tower of London, the Uruguayan-born architect says: "Am I breaking the illusion that we're living in the 13th century? The view from the Tower is already ruined – would it be logical to demolish all of the visible modern buildings?" Rees agrees, rebuffing accusations from what he calls the "heritage Taliban" with a characteristic quip: "I believe it's the Tower of London, not London of the Tower."

The Walkie-Talkie can only be truly judged once complete, but it seems unlikely that the perception of it as a bullying presence in the City will change much – a building with its origins in another more prosperous time, now stranded in the wrong place. Even if it does have a nice view.

Viñoly is indifferent to the criticism, though, seeming to enjoy the controversy his project has created. "As Oscar Niemeyer used to say, 'You can like it or dislike it, but you're not going to forget it.'" It should come as no surprise that, compared to the stifling zoning laws of New York, Viñoly finds London a liberating place to work, hailing the City's planning system as a "truly engaging and positive process"; he describes Rees as "a remarkable bureaucrat … doing something which is really gutsy and totally brave".

He is untroubled by the extent of Rees's powers: "You can say it's a form of absolutism, but at least you have someone to blame, for the good and the bad. If Louis XIV hadn't been the king, then Paris wouldn't have happened – right?"

A new building on London's skyline nicknamed the Walkie Talkie has been blamed for melted car parts due to the intense sunlight reflected from its glass exterior. In a broadcast for Sky News one reporter proves that it is possible to fry an egg in the reflected sunlight. Developers say they are working to rectify the problem